Computers, which support contemporary civilization, operate with current which is a flow of electrons. Devices applied to recording and deletion of information by operation of the current are formed with a semiconductor. Electrons passing through the semiconductor generate Joule heat when they are scattered by impurities or Coulomb force.
Accordingly, a cooling fan is necessary for the computers. The Joule heat causes energy loss since it disables part of input energy from being used for recording and deletion of information. Therefore, there is no doubt that suppressing the electron scattering is a main challenge in technical development for power-saving electronic devices.
As one of the approaches to the challenge, there is conventionally a method for suppressing the electron scattering by operating the electronic devices at cryogenic temperature. For example, use of a superconductor corresponds to the method. Since electron scattering becomes zero in the superconductor, there is no electric resistance nor generation of Joule heat. Therefore, the electron scattering does not occur.
However, it should be noted that the method requires cooling of the electronic devices to the temperature of several kelvins, and execution of this process requires energy. It is also difficult to generalize and commercialize the electronic devices using such a cryogenic temperature state. Accordingly, there is still no sufficient means that can suppress the electron scattering at room temperature.
However, this situation has been changing from around 2007. The change is triggered by a theoretical model of a topological insulator proposed as a theory of physics. The topological insulator, which is an insulator using a special electronic state generated on an object surface or in an interface between the objects, is explained based on the relativistic effect generated when an inner-shell electron of an element having a relatively large atomic number moves at a rate close to the velocity of light.
That is, due to the function (spin-orbit interaction) of this electron, a spin-orbit interaction term is added to a Hamiltonian having a band structure formed by the electron. As a result, a change occurs in the band structure and in an energy eigenvalue. In this case, a certain special substance may have a special band structure in which an uppermost band in the valence band and a lowermost band in the conduction band are bonded on a vacuum surface, while an empty band remains in the inside of the substance.
This results in appearance of an unknown special physical property characterized by formation of a conductor on the surface of the substance or the interface thereof and formation of an insulator in the inside of the substance due to the presence of the band. A substance with such characteristics is called a “topological insulator” (see H. Zhang et al. Nature Physics, 5, 438 (2009)).
The special electronic band structure of the topological insulator has strange characteristics that electrons present on the surface of the substance or the interface thereof are divided into two electron spin currents having different spins due to time-reversal symmetry, and these currents continue to flow without application of voltage. This signifies, in other words, that the topological insulator has an important property of being resistant to the electron scattering caused by the impurities and the like. This characteristic is very strongly preserved if, for example, there is no external magnetic field that compromises the time-reversal symmetry. The name of the topological insulator is originated from the fact that the characteristics of such an electronic band structure are similar to the properties of the topological polyhedron theory in mathematics (see H. Zhang et al. Nature Physics, 5, 438 (2009)).
Since the presence of the topological insulator was theoretically predicted, search for materials having this strange property had started. As a result, materials such as bismuth-tellurium alloys and antimony-tellurium alloys having high crystallinity have been confirmed by experiments performed with photoelectron spectroscopy. However, since single crystals used for these experiments were fabricated by such a method involving cooling of fusion alloys, it is difficult to immediately apply the materials to electronic devices (see Y. Xia et al. Nature Physics, 5, 398 (2009)).
Meanwhile, for reduction in power consumption in phase-change solid-state memories, which do not have any relation to the topological insulator, the inventors of the present invention have proposed a superlattice phase-change solid-state memory including a superlattice phase-change film which is formed from a laminate of a crystal alloy layer made of germanium tellurium and a crystal alloy layer made of antimony tellurium, the layers being laminated such that (111) surface axes of the respective crystal alloy layers are aligned with a c-axis. In this memory, an arraying structure of germanium atoms are switched in a crystal growth axial direction to enable memory operation to be executed (see Japanese Patent No. 4621897, Japanese Patent No. 4635236 and J. Tominaga et al. Nature Nanotechnology, 6, 501 (2011)).
The inventors of the present invention noticed that the superlattice phase-change solid-state memory can be an ideal topological insulator. This is based on the following reason. That is, while a crystal alloy layer (Sb2Te3 crystal alloy layer) having an atomic ratio of antimony and tellurium being 2:3 is defined as the topological insulator as disclosed in H. Zhang et al. Nature Physics, 5, 438 (2009), a structure including the plurality of crystal alloy layers isolated by a crystal alloy layer (GeTe crystal alloy layer) having an atomic ratio of germanium and tellurium being 1:1 and having a band gap is just used as a recording layer of the superlattice phase-change solid-state memory proposed by the inventors of the present invention. Of course, it should be confirmed whether the crystal alloy layer made of germanium and tellurium has the same function as a vacuum band. Accordingly, a simulation was performed based on ab initio calculation using the quantum theory, which confirmed that the crystal alloy layer plays the role equivalent to that of the vacuum band (see International Publication No. WO2013/125101).
According to the simulation, at a certain point (gamma point) in reciprocal space, a lowermost band in the conduction band and an uppermost band in the valence band cross each other in the vicinity of the fermi band so as to be in point contact with each other. This phenomenon is a specific characteristic of the topological insulator, and the gamma point precisely corresponds to a centrosymmetric point of the GeTe crystal alloy layer. That is, this layer serves as a nonscattering layer of the electrons, so that the electrons can freely move in two dimensions in this layer (see International Publication No. WO2013/125101).
As a result of advancing the ab initio calculation with the number of blocks (one block is about 1 nm) of the Sb2Te3 crystal alloy layer being varied, the inventors of the present invention found out that band crossing which is a characteristic of the topological insulator is exhibited when the number of blocks of the Sb2Te3 crystal alloy layer is not one but at least two or more.
It was also found out that layers thinner than 2 blocks do not turn into topological insulators but instead, their band degeneracy is lifted at a gamma point in reciprocal space and they exhibit what is called a Rashba effect of splitting into two spin bands having different energy states.
The Rashba effect exhibited by the superlattice phase-change film is significantly larger than the effect exhibited by any other known materials. A difference between these spin bands reaches as high as 200 meV in the simulation based on the ab initio calculation. The value is so large that the difference in spin characteristics can be observed even at room temperature (see J. Tominaga et al. Applied Physics Letter, 99, 152105 (2011)).
The superlattice phase-change film was also formed on a silicon wafer with a thickness of the Sb2Te3 crystal alloy layer being varied, and an external magnetic field was applied in a direction perpendicular to the surface of the layer to vary the density of split spin electrons. This state was measured as a change in reflectance by making circularly polarized light incident thereon. The result indicated that when the Sb2Te3 crystal alloy layer is thinner than 2 nm, the Rashba effect is notable, whereas when the Sb2Te3 crystal alloy layer is 2 nm or more, a difference in reflectance caused by spin splitting is small. To put it another way, it can be concluded that the superlattice phase-change film, in which the Sb2Te3 crystal alloy layer has the thickness of 2 nm or more, has a small Rashba effect and turns into the topological insulator.
That is, a laminated film made up of the GeTe crystal alloy layer and the Sb2Te3 crystal alloy layer thinner than 2 nm serves as a spin current generation layer having the Rashba effect, whereas a laminated film made up of the GeTe crystal alloy layer and the Sb2Te3 crystal alloy layer having a thickness of 2 nm or more can serve as a spin current accumulation layer which can accumulate a spin current. If these two kinds of crystal alloy layers are laminated and an electric field is applied, for example, in a perpendicular direction for electron injection, it becomes possible to provide a spin electronic memory capable of performing not only spin control but also spin accumulation (see International Publication No. WO2013/125101).
However, when spin accumulation is performed in the spin electronic memory with the technique involving the topological insulator, recording and reproduction of multi-valued information cannot be performed since only two states: a spin liberated state; and a spin accumulated state, can be used. Accordingly, development of a spin electronic memory having a larger memory capacity is being expected.